A typical nuclear steam generator comprises a vertically oriented shell or vessel. A tube sheet adjacent the lower end of the vessel supports a tube bundle comprising a plurality of tubes, each generally in the shape of an inverted U. The portion of the vessel beneath the tube sheet is divided by a partition into inlet and outlet plenums which are respectively disposed immediately beneath the vertical legs of the tube bundle. The vessel is provided with primary fluid inlet and outlet nozzles, respectively communicating with the plenums, each nozzle projecting outwardly of the vessel and commonly having a frusto-conical portion and a cylindrical portion. The nozzles are connected by conduits to an associated nuclear reactor vessel.
Primary fluid having been heated by circulation through the reactor, enters the steam generator vessel through the primary inlet nozzle to the inlet plenum and from there flows upwardly through the tube bundle to the outlet plenum and then back through the outlet nozzle to the reactor core. The tube bundle above the tube sheet is immersed in a secondary fluid, such as water, the tube bundle serving as a heat exchanger for converting the secondary water to steam, which is then used for generating electricity in the usual manner.
Periodically, it is necessary to shut down the reactor for refueling. This is usually a convenient time for servicing the nuclear steam generator. In this regard, the reactor is drained to below the level of the inlet and outlet nozzles of the steam generator vessel. The tube bundle and inlet and outlet plena are also drained and dams are then installed in the inlet and outlet nozzles to block them. The steam generating vessel can then be treated in a chemical decontamination procedure with out contaminating the primary fluid conduits and the nuclear reactor.
Additionally, during refueling operations in a nuclear generating plant, it is desirable to simultaneously perform maintenance procedures on the steam generator. To do this, the inlet nozzle of the steam generator vessel must be closed off. This is accomplished by use of a nozzle seal or dam which is installed in the nozzle from inside the inlet plenum of the generator vessel.
Heretofore, a number of different types of fixed or static nozzle dams have been utilized. Most of these dams involve permanent modification of the nozzle, either by drilling holes therein or the mounting of attachment structure thereon, these techniques entail significant man-rem exposure. It is also known to utilize expandable bags or diaphragms or the like which are inserted in the nozzle and expanded to plug the nozzle. But such prior expandable seals do not adhere particularly well to the nozzle inner surface.
One such nozzle seal is disclosed, for example, in U.S. Pat. No. 4,637,588 issued to Calhoon et al. and entitled "Non-Bolted Ringless Nozzle Dam". Since work must be done within the inlet plenum, the nozzle seal or dam must be fully secured about the nozzle. While the nozzle seal is provided with an anchor assembly for anchoring it in place in the nozzle; nevertheless, this may be insufficient to insure that the nozzle seal will not be dislodged or displaced as a result of large pressure buildups within the nozzle, which could result in the release of reactor coolant which would be extremely hazardous to service and maintenance personnel.
It has been suggested that retention of the nozzle seal be accomplished by a restraint beam which essentially operates like a bar or beam wedged between the nozzle seal and the environmental ledge of the generator vessel. However, such a beam arrangement is quite heavy and difficult to install, resulting in large exposure times. Its construction and manner of installation concentrates all the retaining force at a single location on the steam generator environmental ledge, creating a high risk area at that point. Furthermore, such a device does not readily accommodate manufacturing tolerances in the dimensions of the nozzle and the nozzle seal which result in variations in the seating location of the nozzle seal in the nozzle.
Another version of the Westinghouse nozzle dam incorporates two or more of the circular foldable seal assemblies, interconnected by a central tubular coupling, incorporating a quick-disconnect which is operable from outside the seal assemblies. The coupling is threadedly engaged with each of the two interconnected seal assemblies. While this arrangement operates well in most applications, any torque on either of the two seal assemblies tends to cause relative rotation of the seal assemblies, resulting in unscrewing of the coupling and attendant loosening of the seal. Such loosening may result in excessive leakage about the seal.
Furthermore, the seal assemblies of this arrangement could move as a result of pressure changes on opposite sides thereof, improper seating and the like and, as a result of such movement, the two seal assemblies may not stay parallel, which also would result in poor sealing and resultant leakage. This non-parallel condition could cause the center tubular coupling to be bent, which would interfere with the operation of the quick disconnect mechanism.
In an effort to overcome the aforementioned shortcomings, U.S. Pat. No. 4,671,326 issued to Wilhelm et al. discloses a dual seal nozzle dam with each of the seals comprising a circular three section folded seal plate covered with a flexible diaphragm and circumferentially encompassed by an inflatable seal which when in place frictionally engages the inner surface of the nozzle in order to maintain the nozzle dam in place therein. The dual seals are interconnected by a central tubular coupling. However, as set forth in this disclosure, the positioning and securing of the nozzle dam is carried out by operation personnel thereby exposing such personnel to unnecessary amounts of radioactivity. Moreover, once in place, the inflatable seals may be ruptured thereby causing insurmountable leakage or the complete dislodging of the nozzle dam from within the nozzle.
U.S. Pat. No. 4,684,491 issued to Rylatt and assigned to the assignee of the present application discloses yet another nozzle dam including a retaining assembly which holds the nozzle dam within the nozzle of the nuclear steam generator. Again, this device is positioned within the nozzle of the nuclear steam generator by maintenance personnel who are thus subjected to unnecessary amounts of radiation. Moreover, because the locking mechanism is inserted within a plurality of tubes within the tube sheet, those tubes already accommodated by the locking mechanism will be difficult to inspect and, consequently, may not be maintained in their optimal condition.
U.S. Pat. No. 4,667,701 issued to Evans et al. discloses a multi-section nozzle dam which is hingedly connected to the inner periphery of the inlet nozzle. A frame is secured to the inlet nozzle and the multiple nozzle dam segments are secured thereto. A sealing diaphragm is initially positioned over the uppermost segment and once each section has been secured to the frame, the diaphragm is drawn over the remaining segments. Again, however, as with the previous constructions, the nozzle dam of U.S. Pat. No. 4,667,701 must be manually secured within the plenum of the steam generator and consequently subjects maintenance personnel to significant radiation exposure. Moreover, the frame is continuously exposed to contaminants.
Clearly, there is a need for a simple, reliable nozzle dam for sealing off the nozzles of the channel head of a nuclear steam generator. Moreover, there is a need for a nozzle dam which may be both reliably and remotely installed, positioned and secured over the nozzle of the nuclear steam generator such that maintenance personnel are exposed to minimal amounts of radiation.